Proximity sensors have many different applications in many different industries. In the aerospace industry, for example, proximity sensors can be used within an aircraft to detect the position of various movable components. More specifically, proximity sensors can be used to detect the position of aircraft landing gear, landing gear doors, spoilers, passenger doors, and/or cargo doors. In this regard, such proximity sensors can be used to indicate aircraft conditions such as weight-on-wheels, landing gear up/down, doors open/closed, and/or spoilers stowed/not stowed.
As will be appreciated, proximity sensors can be configured to detect the presence of an object in accordance with a number of different techniques including, for example, variable reluctance, eddy current loss, saturated core, and the Hall effect. In general, a sensor includes a core of a highly-permeable metal and an inductive coil. While the shape of the core can vary, in one typical configuration, the core can comprise a central post and an outer cylindrical wall. The coil is wrapped around the central post and is contained within the outer cylindrical wall.
In accordance with a variable-reluctance proximity detection technique, an external AC current source drives the coil of the proximity sensor to form an electromagnetic circuit with the core. When a permeable and/or conductive object is brought or otherwise moved into the alternating magnetic field caused by the electromagnetic circuit, the reluctance (i.e., air gap resistance) between the object and the proximity sensor changes, or more particularly, decreases. As the reluctance decreases, the inductance of the coils increases. This increase can then be measured to thereby detect the proximity of the object.
The effective range of the sensor is dependent on the amount of change in inductance created by the object as it approaches. The greater the change the faster the sensor can detect the object both in sense of distance between the object and sensor and the time required to measure the inductance difference.
However, eddy currents are created in the core by the electromagnetic circuit. The eddy currents resist change in the magnetic field. This resistance to change reduces the inductance, and thus the signal, caused by the object entering the magnetic field. Furthermore, the eddy currents also provide a resistive loss to the AC circuit. Both the decrease in inductance and increase in resistance contribute to reduce bandwidth of the circuit and increase the time required to make a measurement. Conversely, the lost signal could be used to sense the object at a greater distance.